In production of formic-acid-based pulp, some formic acid remains in the pulp both in a free form and in a chemically bound form. Formic acid bound chemically to the pulp is typically in the form of a cellulose formate. Bound formic acid is also found esterified in lignin. Chemical binding of formic acid takes place particularly in connection with cooking, evaporating, distilling and drying stages. In general, chemically bound formic acid is formed in the pulp and lignin in an amount of several per cents, for instance, 3 to 5% per dry pulp unit when strong formic acid with a concentration of 60 to 100% by weight is used in cooking.
Washing with water is most generally used to remove formic acid from the pulp produced by the formic acid method. However, mere washing with water is not sufficient to remove the bound formic acid or to reduce its amount to acceptable levels. The acceptable amounts of chemically bound formic acid in a dry, unbleached pulp washed from formic acid are in the order of less than 1%, preferably less than 0.5%.
Finnish Patent 85,510 discloses a previously developed method for removing bound formic acid. This method employs vacuum evaporation at an elevated temperature, whereafter the pulp is washed with hot water or stripped with steam. The temperature used in vacuum evaporation is typically about 70 to 100.degree. C. and in steam stripping about 100 to 140.degree. C. In the vacuum evaporation stage the fonnic acid concentration is very high, whereby the amount of bound formic acid further increases. This bound formic acid is then removed at an elevated temperature with hot water. Drawbacks with the method are that its implementation is difficult, for instance, due to a vacuum evaporator required, and its energy efficiency is poor. High washing temperature, exceeding 100.degree. C., also requires a special device for removing the pulp from the equipment and a unit for treating the vapours that are generated. This method requires additional investments and consumes a great quantity of energy, since the operations are carried out at higher temperatures than other process stages. Also, it is to be noted that high washing temperatures, exceeding 100.degree. C., may have a deteriorating effect on the fibre quality, for instance, on strength properties.
The object of the method of the invention is to eliminate the drawbacks of the prior art methods and to provide a method that is useful in industrial processes for removing chemically bound formic acid without having to use high temperatures exceeding 100.degree. C. and with using normal pressure.